Television transmitter



oct. 13,Y 1942.

R. D. KELL TELEVISION TRANSMITTER Filed Sepi. 30, 1939 2 Sheets-Sheet/ 1 knllbh.

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Zhm'enfor Ray D, KeZL Gttomeg 0t.13,1942. y R KELL `2,298,797

TELEVISION TRANSMITTER "Filed sept. so, 1939 2 sheets-sheet 2- ]ira. Z.-

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MI Gttomeg Patented Oct. 13 1942 TELEVISION TRANSMITTER Ray Kell, Haddoniield, N. J., assignerto Radio Corporation of America, a corporation of Dela- Application September 30, V1939, Serial No. 297,216

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4 Claims.

My invention relates to television transmitters, and particularly to television systems wherein the carrier wave is negatively modulated by the video signal.

By negative modulation is meant modulation in such direction that a decrease in light in the picture results in an increase in the carrier wave amplitude. Thus,` in the usual case where synchronizing pulsesare in the direction representing black inthe signal, Vthese pulses are represented by maximum carrier amplitude.

It is an object of my invention to provide in a system of the above-described type an improved meansfor and method of operating a television transmitter.

More specifically, an object of my invention is to obtain maximum power from a television transmitter without the risk of damage to the power amplier in the event of interruption of the video signal.

In accordance with my invention, the television transmitter is designed to transmit the direct current component of the video signal whereby the peaks of the synchronizing impulses are at a fixed peak carrier wave amplitude and these synchronizing impulses are caused to drive the transmitter to its maximum power output. This maximum power is more than the transmitter can radiate continuously, the limit being determined by the plate dissipation in the nal radio frequency amplifier, but it can radiate this power intermittently. Between successive synchronizing impulses, the power drops at least to the level representing black in the picture and, if picture signal is on, it drops substantially below the black level, Varying between the low level representing white in the picture and the high level representing blackf Further, in accordance with my invention, the transmitter is so designed that, if the picture signal and synchronizing impulses are interrupted so that they are no longer modulating the carrier wave, suitable protective means automatically becomes effective to hold the carrier amplitude to a safe level. In a preferred embodiment, the automatic protective means decreases the negative bias on the modulator tube in response to such interruption which, in turn, increases the negative bias on the direct current coupled power amplier.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a. circuit diagram of a television transmitter embodying the invention,

Figure 2 is a curve which is referred to in explaining the operation'of the apparatus shown in Figure 1, and

Figure 3 is a curve representing the video signal which is applied to the protective relay circuit. 1

Referring to Fig. 1, the invention is shown applied to a ltelevision transmitter comprising the necessary terminal equipment and amplifiers indicated by the block I0 for supplying the video signal which is to modulate the transmitted carrier wave. This video signal preferably is of the character shown in Fig. 3 and comprises the picture signal I3 and regularly recurring line synchronizing pulses II set on pedestals I2. The video signal also includes regularly recurring framing pulses not shown.

The terminal equipment I0 may be the same as that described in Bedford British Patent 448,065 and in the corresponding U. S. Patent 2,192,121, issued Feb. 27, 1940, one important feature of the circuit being that the pedestals I2 are caused to go to a xed level such as black in the picture, whereby the direct current component of the picture signal may be reinserted after the signal has passed through alternating current amplifiers.

Or the terminal equipment I0 may be substantially the same as that shown in Vance Patent No. 2,137,039, issued November 15, 1938. In the Vance system, the peaks of the synchronizing pulses go to black or to a xed level beyond black The video signal is amplified by suitable amplifiers I6 and II and supplied to a modulator tube I8. l The plate circuit of the tube I8 is direct current coupled to a push-pull power amplier I9 for modulating the radio frequency carrier supplied from a suitable carrier wave source 2l.

The power ampliiier I9 comprises the amplier tubes 22 and 23 which have a certain output rating determined by the plate dissipation as previously mentioned.

The direct current coupling between the modulator I8 and the amplifier I9 preferably is of the type described in Blumlein British Patent 462,530, but for the sake of simplicity is shown in` the drawing as of a conventional type including a battery 26.

Since the direct current component of the picture signal isto be transmitted, it is inserted, in the example illustrated, at the input of the modulator tube I8 by means of a diode 21. At the grid of the tube I8 and, therefore, at the cathode of the diode 21, the synchronizingpulses are negative whereby they cause a flow or pulse of current through the diode each time they occur. These pulses of current charge the coupling condenser 28, which discharges comparatively slowly through the grid leak resistor 29. The time constants of the charging and discharging circuits for condenser 28 as such that it holds over most of its charge between successive line synchronizing pulses, whereby it applies a bias voltage to the grid of tube I8 which is a measure of the height of the synchronizing pulses from the A. C. axis and, therefore, a measure of the D. C. component of the signal. This diode type of circuit for reinserting the D. C. component is described in Willans Britishy Patents 422,906 and 422,914.

As previously indicated, the transmitter radiates maximum power when no modulating signal is being supplied (ignoring, for the present, the action of the protective circuit). This result is obtained by applying to the grid of tube IS from the voltage divider 44 a large negative bias which biases tube |8 close to cut-01T and which puts such bias on the grids of the power amplier tubes 22 and 23 that there is maximum power output.

When the video signal is supplied to the transmitter,`the D. C. reinserting diode 21 and associated circuit cause a bias voltage to appear across the grid'leak resistor 29 which opposes the i'ixed negative' bias from the voltage divider 44. The difference in these two biases is the working bias ci the tube I8 and the peaks of the synchronizing pulses are held at a xed point close to cut-off on the operating characteristic of tube I8. Consequently, at the amplier I9, the synchronizing peaks are held at a iixed point on the operating characteristics of the tubes 22 and 23 such that maximum power output is obtained for the duration ofeach synchronizing pulse. More specifically, the bias Voltage from the battery 26 is made such that the amplier I 9 is so biased that it operates substantially class B. This means that, when there is no video signal input, the grids of tubes 22 and 23 may have nearly zero bias thereon.

It will be understood that, in place of the diode 21 and associated circuit, any other suitable D. C. reinserting means may be employed.

The radiated carrier, as modulated by the video signal including the D. C. component, is of the character shown in Fig. 2. It will be seen that the synchronizing pulses are represented by the maximum carrier amplitude and that if their transmission to the modulator is interrupted, the carrier wave will go to maximum amplitude and remain there in the absence of protective means for reducing the carrier amplitude in response to such interruption.

In accordance with my invention, such protective means is provided. It should be understood that the specic protective means which will now be described is merely by way of example. Referring to Fig. 1, the protective circuit may comprise a vacuum tube 3| having a cathode 32, a grid 33, an anode 34 and a diode plate 36.

The video signal, of the character shown in Fig. 3, is applied through a resistor 31 and a condenser 38 to the diode unit 36-32 of the tube 3| with the synchronizing pulses of positive p0- larity whereby unidirectional current pulses are applied to a filter section comprising a resistor 39 and a condenser 4|, This operation of the protective relay circuit may be described as follows:

The synchronizing pulses cause a flow of current through the condenser 38 and diode 36-32 in series whereby condenser 38 receives a charge which applies negative voltage to the grid 33. This charge holds over between successive horizontal synchronizing pulses since the discharge path is through a resistor 35 having enough resistance to make the discharge path for condenser 33 have the required time constant.

Since the control grid 33 is connected to the diode section 36-32 through the iilter 394I, it has applied thereto a comparatively steady negative'; voltage, and little or no current iiows through a relay coil 42 connected in the plate circuit of the tube 3|.

Under these conditions, the contact points 43 of the relay are closed whereby they short-circuit the right-hand section of a voltage divider 44 with the result that there is maximum current flowing through the left-hand section of the voltage divider 44 and maximum negative bias is applied to the grid of modulator tube I8. Consequently, because of the D. C'. coupling between modulator I8 and power amplifier I9 there is minimum negative bias applied to amplifier I9 and the synchronizing pulses drive the carrier to the maximum amplitude indicated in Fig. 2. The amplifier I9 can supply a carrier wave of this maximum amplitude intermittently but not continuously.

In the event of failure of the video signal, that is, if there is no longer either picture signal or synchronizing pulses applied to the protective tube 3|, the grid 33 is no longer held negatively biased and plate current ows through the relay coil 42 to open the contact points 43. This reduces the negative bias on the modulator tube grid and increases it on the grids of power ampliiier I9 whereby the carrier amplitude is reduced to a safe value such as to the amplitude representing "black instead of being permitted to go to the amplitude indicated in Fig. 2 by the legend Video signal 01T.

It will be noted that the operating voltage is supplied to the plate of tube 3| through the usual lter comprising la resistor 46 and condensers 41 and 48 and that a smoothing condenser 49 is connected in shunt to the relay coil 42. I'he usual resistor 5| and condenser 52 are connected across the contact points 43 for reducing sparking.

I claim as my invention:

l. The method of transmitting television pictures from a radio transmitter including a final radio frequency amplier stage having a certain output rating for continuous output, said method comprising producing a carrier wave, producing a composite signal comprising picture signals and periodically recurring synchronizing impulses which impulses are in the direction of black in the picture being transmitted, inserting in said composite signal the direct current component of said picture, modulating said carrier wave negatively with the resulting composite signal and with the synchronizing impulses driving said am-` plier stage substantially to maximum power output and substantially above said certain output rating, and automatically reducing and holding the output of said amplifier stage to at least substantially said continuous output rating in response to interruption of said composite signal.

2. In a system for transmitting television Ypictures from a radio transmitter including a final radio frequency amplifier stage having a certain output rating for continuous output, means for producing a carrier wave, means for producing a composite signal comprising picture signals and periodically recurring synchronizing impulses which impulses are in the direction of black in the picture being transmitted, means for inserting in said composite signal the direct current component of said picture, means for modulating said carrier Wave negatively with the resulting composite signal and with the synchronizing impulses driving said amplier stage substantially to maximum power output and substantially above said certain output rating, and means for automatically reducing the power output of said amplifier stage in response to said carrier Wave becoming unmodulated.

3. In a system for transmitting television pictures from a radio transmitter including a nal radio frequency amplifier stage having a certain output rating for continuous output, means for producing a carrier wave, means for producing a composite signal comprising picture signals and periodically recurring synchronizing impulses which impulses are in the direction of black in the picture being transmitted, means for inserting in said composite signal the direct current component of said picture, means for modulating said carrier wave negatively with the resulting composite signal and with the synchronizing impulses driving said amplifier stage substantially to maximum power output and substantially above said certain output rating, and means for automatically holding the power output of said amplifier stage substantially below said maximum power output in response t0 interruption of said composite signal.

4. A television transmitter comprising a radio frequency amplifier, a source of carrier wave voltage, a modulator tube having a control grid, direct current coupling between said modulator and said amplifier, means for applying a modulating signal to said control grid and means for applying said carrier wave voltage to said amplifier whereby a modulated carrier wave is produced, a source of substantially constant negative bias, means for producing a positive bias which is a measure of the direct current component of the picture being transmitted, means for applying said biases in additive relation to said control grid whereby minimum negative bias is applied to said amplifier in the absence of a.

modulating signal, and protective means for decreasing the negative bias on said control grid in response to interruption of the modulating signal, said decrease in bias being sufficient to prevent the power amplifier from exceeding its maximum power rating for continuous output. RAY D. KELL. 

